The present invention relates to an apparatus for cleaning a filter cloth in a filtering device.
Filtering devices of the type stated by way of introduction are used especially for filtering of particles from liquids.
Two variants of filtering devices of this type are predominant on the market.
The one variant is in technical language called rotary disc filter and is described, for example, in the brochure xe2x80x9cSkivfilterxe2x80x9d (in English: Rotary Disc Filter) issued by the Applicant (Hydrotech, 1996). The brochure, p. 2, shows in a perspective view the principle of such a rotary disc filter, see FIG. 1. The rotary disc filter 10 comprises a horizontal, slowly rotating drum 11, which supports a number of parallel, disc-shaped filter elements 12 which are successively arranged in spaced-apart relationship along the centre axis of the drum and which extend radially away from the drum 11. The filter elements 12 have on their opposite lateral surfaces relatively large openings which are covered with a filter cloth 13. The drum 11 has a number of openings through which the liquid A which is to be filtered can flow to the interior of the filter elements 12. The filtering liquid A is conducted through an inlet into the drum 11 and then falls down to the interior of the disc-shaped elements 12, from which the liquid A continues out through the filter cloth 13 so that the particles which are to be fiitered off adhere to the inside of the filter cloth 13. For cleaning of the filter cloth 13, the disc filter 10 is provided with spray nozzles 14 which are mounted on a number of fixed flush tubes 15, which extend between the disc-shaped filter elements 12. The flush tubes 15 are adapted to conduct flushing liquid to the spray nozzles 14 and are connected to an elongate liquid-conducting pipe 16 which extends parallel with the centre axis of the drum 11. The spray nozzles 14 spray the filter cloth 13 axially from the outside, in a direction opposite to the filtering direction, and the flushed-away particles are collected in a discharge trough 17 which is placed inside the drum 11 in the upper portion thereof.
The other variant is in technical language called drum filter and described, for example, in the brochure xe2x80x9cTrumfilterxe2x80x9d (in English: Drum Filter) issued by the Applicant (Hydrotech, 1996). The brochure, p. 2, shows in a perspective view the principle of such a drum filter. See FIG. 2 where the drum 21 has been cut open to be able to show the flushing equipment arranged outside the drum 21. Thus the drum filter 20 comprises a horizontal, slowly rotating drum 21, whose circumferential surface has relatively large openings which are covered with a filter cloth 22. The liquid A that is to be filtered is passed into the drum 21 and is filtered as it falls downwards through the filter cloth 22 and out of the drum 21. To avoid clogging of the filter cloth 22 on the rotating drum 21, this is flushed clean with the aid of fixedly mounted spray nozzles 23 which are placed on the outside of the drum 21 and above the liquid level inside the drum 21. The flushing is carried out radially inwards in a direction opposite to the filtering direction. The dirt which is flushed away from the cloth 22 inwards in the drum 21 reaches a discharge trough 24 which is placed inside the drum 21 just above its centre axis and which draws off the dirt.
The reverse flushing or cleaning can in both cases occur with a rotating or stationary drum, and the flushing may occur continuously or intermittently.
A problem which is common to these filtering devices is that the filter cloth has a tendency to clog in spite of the particular spray nozzles which flush the filter cloth in a direction opposite to the filtering direction. Clogging of the filter cloth deteriorates the efficiency or degree of efficiency of the filtering device.
Moreover the filter cloth of both variants has a tendency to be xe2x80x9cstripedxe2x80x9d owing to irregular cleaning, which is a sign of the filter cloth not being sufficiently cleaned in certain portions.
The above drawbacks in turn result in the filter cloth needing be replaced too frequently.
For satisfactory cleaning of the filter cloth, a plurality of spray nozzles must according to the prior art solutions be arranged extremely close together, which means that the filtering device is rendered expensive.
An object of the present invention is suggest a solution to the above problems by providing an improved apparatus for cleaning a filter cloth in a filtering device.
The cleaning apparatus according to the invention comprises a driving device for providing a reciprocating motion of the spray nozzles. The reciprocating motion allows each spray nozzle to clean a larger portion of the filter cloth, which results in more efficient cleaning. Besides, the number of spray nozzles can be reduced.
The driving device has a coupling which is adapted to transform the rotary motion of the drum into the reciprocating motion of the spray nozzles. The coupling has such a gear ratio that the rotary motion of the drum and the reciprocating motion of the spray nozzles are out of phase with each other. By these two motions not occurring in step or phase with each other, it is avoided that a certain spray nozzle flushes the same parts of the filter cloth revolution after revolution. This means that each portion of the filter cloth is once now and then subjected to direct flushing by a spray nozzle. If the motions had occurred in phase, the above-described xe2x80x9cstripedxe2x80x9d condition of the filter cloth would only have been converted into curved lines instead, thus causing incomplete cleaning.
The reciprocating motion of the spray nozzles preferably has a motion vector component, which is in a plane parallel to that portion of the filter cloth which in a certain rotational position is flushed and which is directed perpendicular to the direction of motion of said filter cloth portion. By the filter cloth moving in one direction and the spray nozzles at least partially moving in a direction perpendicular thereto, all portions of the filter cloth can be reached by a flushing jet from a spray nozzle.
The spray nozzles are advantageously arranged on a flush tube which is adapted to conduct the cleaning liquid to all spray nozzles. This solution makes the construction compact and robust.
According to a first aspect of the invention, the filtering device further comprises at least one disc-shaped filter element, whose normal direction is parallel and essentially concentric with the centre axis of the drum. The filter element extends radially away from and round the drum. The filter cloth is supported on at least one of the lateral surfaces of the disc-shaped filter element. The cleaning apparatus according to this first aspect advantageously comprises a flushing, ramp which is pivotable about a pivot axis, which is parallel with the centre axis of the drum. The pivotability of the flushing ramp gives the spray nozzles a reciprocating rocking motion, which results in more efficient cleaning of the filter cloth and, thus, an improved degree of efficiency of the filtering device.
According to a preferred embodiment of this aspect, the flushing ramp has flush tubes each supporting at least one spray nozzle. The flush tubes are intended to conduct the cleaning liquid to the spray nozzles. Thanks to the rocking motion of the flushing ramp, a spray nozzle supported on a flush tube is given a reciprocating motion on the one hand in the radial direction and, on the other hand, in the tangential direction, thus allowing all portions of the filter cloth to be reached by a spray nozzle.
For utilisation of the limited space between the disc-shaped elements, the flush tube is preferably arranged in a plane which is essentially parallel with the lateral surface of the filter element.
The flush tube has such a radial extent as to extend along essentially the entire portion of the lateral surface of the disc-shaped element which is provided with filter cloth. This allows all portions of the filter cloth to be reached by a spray nozzle supported on the flush tube, without the flush tube having to make a great and inconvenient motion.
According to a preferred embodiment of the invention in its first aspect, said coupling between the rotary motion of the drum and the reciprocating motion of the spray nozzle is provided by means of a loop-shaped drive between an eccentrically mounted wheel connected to the flushing ramp and a wheel non-rotationally mounted on the rotary shaft of the drum. Since the loop-shaped drive has an essentially constant length and the wheel connected to the flushing ramp is eccentrically mounted, the flushing ramp is forced to make said reciprocating rocking pivoting motion.
To ensure that the rotary motion of the drum and the reciprocating motion of the flushing are out of phase, the wheels have an uneven, ratio-generating ratio of diameters. For example, the two diameter dimensions can be non-divisible by each other.
According to a second aspect of the invention, the filter cloth is supported on the circumferential surface of the drum, at least one spray nozzle being arranged on an elongate flushing ramp with an elongate flush tube which is intended to conduct cleaning liquid to the spray nozzles and which is arranged essentially parallel with the centre axis of the drum. This solution makes the construction compact and robust.
The flush tube is preferably reciprocable along its longitudinal axis for providing the reciprocating motion of the spray nozzles, which results in more efficient cleaning of the filter cloth and, thus, a higher degree of efficiency of the filtering device.
According to a preferred embodiment of this second aspect, a drive wheel is arranged on a drive shaft which via said coupling rotationally drives the drum. Advantageously the drive wheel has on its circumferential surface a circumferential guide means, and the cleaning apparatus further comprises a follower means connected to the flush tube. To provide the reciprocating motion of the spray nozzles, the follower means engages with and follows the guide means.
The guide, means preferably comprises a wave-shaped groove formed in the circumferential surface of the drive wheel, and the follower means advantageously comprises a pin fixedly connected to the flush tube.
By using a guide means and a follower means, which according to a preferred embodiment consist of a groove and a pin, a motion-transferring coupling is obtained, which is simple, robust and reliable.
To achieve the condition that the rotation of the drum and the reciprocating motion of the flushing ramp are out of phase with each other, the drive shaft supporting the drive wheel is connected to the rotary shaft of the drum with a non-uniform gear ratio. This non-uniform gear ratio causes a spray nozzle not to flush the same portion of the filter cloth revolution after revolution as the drum rotates.
According to a preferred embodiment of one of the two aspects of the invention, the cleaning apparatus comprises at least two spray nozzles, two neighbouring spray nozzles being arranged to spray, in a certain rotational position of the drum, cleaning liquid onto a first and a second portion of the filter cloth. The reciprocating motion of the spray nozzles has a stroke which is at least equal to the distance between said first and second portion of the filter cloth. This stroke causes all portions of the filter cloth to be reached by direct flushing by means of one of the spray nozzles.